1. Field of the Invention
The present invention relates to an optical density-changing element which can undergo a change in optical density in response to an applied voltage, and which has excellent properties of, for example, requiring only a small applied voltage to become in a colored state, responding at a high speed, and being completely bleached when restored to a bleached state from the colored state. In particular, the present invention relates to an electrochromic element; an optical element comprising the optical density-changing element and an electromotive force-generating element capable of generating an electromotive force upon irradiation with an electromagnetic wave; and a photographic unit using the optical element.
2. Background Art
Elements capable of undergoing a change in optical density in response to an electromagnetic wave find wide applications. Materials having the function of changing optical density in response to the electromagnetic wave, i.e., controlling transmission or reflection of light include photochromic materials, electrochromic materials and thermochromic materials.
The photochromic materials are materials capable of undergoing the change in optical density thereof upon irradiation with light, and have found applications to sunglasses, UV checkers, printing-related materials and fibrous processed products. However, these materials show only a slow response speed, and their use has been limited. Materials with an accelerated response speed are expected to find wider applications such as materials for vehicle window, display devices and camera-related optical elements.
Electrochromic materials are materials capable of undergoing the change in optical density thereof upon release or receipt of electrons, and have found applications to display devices, automatically dimmable rear-view mirrors for automobiles and window materials for vehicles. Electrochromic materials as display elements are expected to be more and more developed to realize large area displays, displays with a higher response speed, displays with a higher density and displays with a higher quality. In comparison with the photochromism, electrochromism provides a higher response speed, and hence there exists a high possibility of its application to not only display elements but camera-related optical elements as well.
The thermochromic materials are materials capable of undergoing the change in optical density thereof upon temperature change, and have found applications to toys, medicines and foods. Particularly in recent years, they have realized packaging members with a temperature-indicating function. In comparison with the electrochromism, however, thermochromism is slow in response speed and low in light transmission, and hence its use is limited, and its application to window materials for vehicles, display devices or camera-related optical elements involves difficulty.
The use of such optical density-changing material includes a photographic system such as a camera. For example, in recent years, a film-with-lens unit has come into wide use due to its handiness as a camera unit which eliminates the necessity of film-loading operation and enables one to take pictures immediately after buying it. In order to enhance its utilization values, development has become more and more active in recent years for enlarging a photographing range by loading a fast film. However, since conventional film-with-lens units whose advantage is handiness are not equipped with an exposure-controlling mechanism, film-with-lens units loaded with a fast film often cause overexposure in a high intensity region, and thus their weak point of “being difficult to use” has been revealed. Therefore, film-with-lens units wherein AE control system of measuring a light amount during photographing is introduced and which can automatically changing aperture depending upon the amount of photographic light have been placed on the market. Thus, frequency of so-called “unsuccessful photographs” due to the overexposure has been greatly reduced.
There have been proposed film-with-lens units using, as means for realizing a “light amount control filter” capable of controlling the incident light amount to a light-sensitive material according to the photographic light amount with more ease at an inexpensive cost, a compound which becomes colored or bleached corresponding to ON or OFF of irradiation with light, so-called “photochromic compound” (see, for example, JP-A-5-142700, JP-A-6-317815 and JP-A-2001-13301).
The photochromic compound is a compound which becomes colored, i.e., shows an increased optical density when irradiated with light, and becomes bleached, i.e., shows a decreased optical density when irradiation is stopped or when heated or irradiated with another light different in wavelength. As such compound, there are known silver halide-containing inorganic compounds and some organic compounds. It has been considered that light amount can be controlled by placing a filter made of the photochromic compound on the optical axis and making the filter colored or bleached according to the incident light amount.
However, it takes generally about one minute for the photochromic compound to become colored and takes longer than several ten minutes to become bleached (see, for example, Solid State and Material Science, 1990, 16, p. 291), and thus it has been difficult to use the compound for a light amount control system for controlling the photographic light amount.
In contrast, as materials which can be colored or bleached at a higher speed, there are illustrated “electrochromic compound” that is colored or discolored upon release or receipt of electrons. The electrochromic compound is a compound which shows an increased optical density upon release or receipt of electron by applying a voltage, and shows a decreased optical density upon electron transfer in reverse direction to the direction on increase in optical density, and it has been known that some metal oxides and organic compounds show this property.
A light amount control system has been proposed wherein a solar cell capable of generating electromotive force in response to light is layered on the electrochromic compound (see, for example, JP-A-9-244072). With this system, too, automatic light amount control according to the amount of light can be expected. However, it is unavoidable, with the structure wherein the solar cell is layered on the electrochromic compound layer as in the above-mentioned proposal, for part of light passing through the electrochromic compound layer to be absorbed by the solar cell. Therefore, it is inappropriate for a system which utilizes transmitted light as much as possible when photographing a scene not requiring light amount control, such as camera-related optical elements in particular.
On the other hand, as a particularly advantageous method for utilizing the electrochromic compound, a method of utilizing a nanoporous electrode wherein a material undergoing a change in optical density as a result of release or receipt of electrons is chemisorped on a nanoporous material is known. It is disclosed that an electrochromic element showing a rapid response speed can be produced by using such electrode (see, for example, JP-T-2000-506629, JP-T-2003-511837 (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.) and Solar Energy Materials and Solar Cells, 1998, 55, p. 215). As to the kind of the nanoporous material, JP-T-2000-506629 discloses a semiconductor material such as titanium oxide, and JP-T-2003-511837 discloses a metal oxide doped with impurity to metallic levels.
Also, an electrochromic element showing a higher response speed at becoming in a colored or bleached state by using a porous electrode of antimony-doped tin oxide having a less resistance than tin oxide has recently been reported (see, for example, Journal of Physical Chemistry B, 2000, 104, p. 11449).
As a result of intensive investigations on the electrochromic element utilizing the nanoporous electrode wherein (i) a material capable of releasing or receiving electrons (a material undergoing or not undergoing a change in optical density in a wavelength range of 400 to 700 nm as a result of release or receipt of electrons) is adsorbed on (ii) a nanoporous material, it has been found that the electrochromic element can show unfavorable performance depending upon combination of (i) material capable of releasing or receiving electrons and (ii) nanoporous material. The term “unfavorable performance” as used herein means that a more voltage is required for making the electrochromic element colored, that the electrochromic element shows a slow response speed, or that the electrochromic element is not made completely bleached when restored to the bleached state, and some of the color remains.
Also, in using for the automatic light amount control system such as that for controlling photographic light amount in a camera mentioned hereinbefore, an electric source for generating an appropriate electromotive force according to the light amount to control operation of the electrochromic element is necessary in addition to the electrochromic element.
It is naturally desired for the electrochromic element to require a small voltage for bringing it into a colored state, to show a rapid response speed, and to be completely bleached when restored to the bleached state from the colored state. It is eagerly desired not to deteriorate performance of the electrochromic element.